U.S. patent application number 10/095830 was filed with the patent office on 2004-02-26 for eletrophoretic display device.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to De Boer, Dirk Kornelis Gerhardus, Johnson, Mark Thomas.
Application Number | 20040036951 10/095830 |
Document ID | / |
Family ID | 8180013 |
Filed Date | 2004-02-26 |
United States Patent
Application |
20040036951 |
Kind Code |
A1 |
Johnson, Mark Thomas ; et
al. |
February 26, 2004 |
Eletrophoretic display device
Abstract
In electrophoretic displays, grey values are realized by
introducing a further electrode (6') in addition to the
conventional electrodes (6, 7)for bistable operation.
Inventors: |
Johnson, Mark Thomas;
(Eindhoven, NL) ; De Boer, Dirk Kornelis Gerhardus;
(Eindhoven, NL) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
|
Family ID: |
8180013 |
Appl. No.: |
10/095830 |
Filed: |
March 12, 2002 |
Current U.S.
Class: |
359/296 ;
345/107 |
Current CPC
Class: |
G02F 1/167 20130101;
G09G 2300/08 20130101; G09G 2310/0275 20130101; G09G 2310/06
20130101; G09G 3/3446 20130101; G02F 1/16757 20190101; G09G 3/2014
20130101; G09G 3/344 20130101; G09G 2310/068 20130101; G09G
2310/061 20130101; G02F 1/16766 20190101; G09G 2310/0205 20130101;
G09G 2320/04 20130101; G02F 1/16762 20190101 |
Class at
Publication: |
359/296 ;
345/107 |
International
Class: |
G02B 026/00; G09G
003/34 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2001 |
EP |
01200952.8 |
Claims
1. An electrophoretic display device comprising at least one pixel
with an electrophoretic medium, and two switching electrodes, as
well as drive means via which the pixel can be brought to different
optical states, wherein the pixel comprises at least one further
electrode and drive means for realizing intermediate optical states
via electric voltages.
2. An electrophoretic display device as claimed in claim 1, wherein
the display device comprises means for bringing the pixel to a
defined state, prior to selection.
3. An electrophoretic display device as claimed in claim 1, wherein
the electrophoretic medium is present between two substrates each
comprising a switching electrode, at least one of the substrates
being provided with the further electrode.
4. An electrophoretic display device as claimed in claim 1 or 2,
wherein the electrophoretic medium is present in a
microcapsule.
5 . An electrophoretic display device as claimed in claim 1 or 3,
wherein pixels are mutually separated by a barrier.
6. An electrophoretic display device as claimed in claim 1, wherein
the electrophoretic medium is present between two substrates, one
of the substrates comprising the switching electrodes and the
further electrode.
7. An electrophoretic display device as claimed in claim 3 or 6,
wherein the switching electrodes are comb-shaped and
interdigital.
8. An electrophoretic display device as claimed in claim 3 or 6,
wherein the further electrode is separated from the switching
electrodes by a layer of dielectric material.
9. An electrophoretic display device as claimed in claim 8, wherein
parts of the further electrode are situated between the teeth of
the two switching electrodes.
10. An electrophoretic display device as claimed in claim 1,
wherein the electrophoretic medium is present in a prismatic
structure.
11. An electrophoretic display device as claimed in claim 10,
wherein the prismatic structure is provided proximate to its base
with the two switching electrodes, and the further electrode is
situated proximate to the top of the prismatic structure.
Description
[0001] The invention relates to an electrophoretic display device
comprising at least one pixel with an electrophoretic medium, and
two switching electrodes, as well as drive means via which the
pixel can be brought to different optical states. Where a switching
electrode is mentioned in this application, it may be divided, if
desired, into a plurality of sub-electrodes which are supplied with
one and the same voltage either externally or via switching
elements.
[0002] Electrophoretic display devices are based on the motion of
charged, usually colored particles under the influence of an
electric field between two extreme states having a different
transmissivity or reflectivity. With these display devices, dark
(colored) characters can be imaged on a light (colored) background,
and vice versa.
[0003] Electrophoretic display devices are therefore notably used
in display devices taking over the function of paper, referred to
as the "white paper" applications (electronic newspapers,
electronic diaries).
[0004] In the known electrophoretic display devices with an
electrophoretic medium between two switching electrodes, the
switching electrodes are supplied with drive voltages. The pixel
may then be brought exclusively to two extreme optical states. One
of the switching electrodes is then realized, for example, as two
mutually interconnected narrow conducting strips on the upper side
of a display element. At a positive voltage across this switching
electrode with respect to a bottom electrode covering the entire
bottom surface of the display element, charged particles
(negatively charged in this example) move to the potential plane
which is defined by the two interconnected narrow conducting
strips. The (negatively) charged particles spread across the front
face of the display element (pixel) which then assumes the color of
the charged particles. At a negative voltage across the switching
electrode with respect to the bottom electrode, the (negatively)
charged particles spread across the bottom face so that the display
element (pixel) assumes the color of the liquid.
[0005] In practice, there is an ever increasing need for displaying
intermediate optical states (referred to as grey values). Known
methods of introducing grey values are usually not satisfactory.
For example, electrophoretic display devices are too slow to
introduce grey values via time-weighted drive periods (time ratio
grey scale). Division of the pixel into different surfaces (area
ratio grey scale) usually requires barriers between the different
sub-pixels so as to prevent mutual crosstalk.
[0006] It is an object of the present invention to meet this
drawback. In an electrophoretic display device according to the
invention, grey values (intermediate optical states) are introduced
by providing the pixel with at least one further electrode and
drive means for supplying the further electrode with electric
voltages.
[0007] The invention is based on the recognition that the electric
field within a display cell can be influenced by means of electric
voltages on the further electrode in such a way that, in the
example described above, the electric field lines at a positive
voltage across the switching electrode with respect to the bottom
electrode are disturbed in such a way that the negatively charged
particles move towards a portion of the surface between the two
electrodes. Dependent on the electric voltages across the switching
electrodes and one (or more) further electrode(s), more or fewer
particles move towards the surface between the two electrodes and
different intermediate optical states (grey values) are
obtained.
[0008] To obtain a satisfactory distribution across the surface
between the two electrodes when the settings are changed, it is
preferred to spread the charged particles uniformly across the
other electrode in advance, for example, by bringing the pixel to a
defined state prior to selection, for example, by giving a reset
pulse, if necessary in combination with a small alternating field
component.
[0009] In a first embodiment, the electrophoretic medium is present
between two substrates, each of which is provided with a switching
electrode, while at least one of the substrates is provided with
the further electrode. The charged particles may then be present in
a liquid between the substrates, but it is alternatively possible
that the electrophoretic medium is present in a microcapsule. In
the first-mentioned case, the pixels may be mutually separated by a
barrier.
[0010] In a further embodiment, the electrophoretic medium is
present between two substrates, one of the substrates comprising
the switching electrodes and the further electrode, notably when
use is made of a lateral effect as described in "Development of
In-Plane EPD", SID 2000 Digest, pp. 24-27.
[0011] In a preferred embodiment, the switching electrodes are
comb-shaped and interdigital, and parts of the (insulated) further
electrode are situated between the teeth of the two switching
electrodes. Alternatively, the electrophoretic medium may be
present in a prismatic structure as described in "New Reflective
Display Based on Total Internal Reflection in Prismatic
Microstructures", Proc. 20.sup.th IDRC conference, pp. 311-314
(2000).
[0012] These and other aspects of the invention are apparent from
and will be elucidated with reference to the embodiments described
hereinafter.
[0013] In the drawings:
[0014] FIG. 1 shows diagrammatically a display device,
[0015] FIG. 2 shows a pixel of an electrophoretic display device
according to the invention, in which different grey values
(intermediate optical states) have been realized,
[0016] FIG. 3 shows the electric field variation in pixels of an
electrophoretic display device according to the invention, for
elucidating the invention,
[0017] FIG. 4 shows another electrophoretic display device
according to the invention, in which different grey values
(intermediate optical states) have been realized,
[0018] FIG. 5 is a plan view of a part of another electrophoretic
display device according to the invention,
[0019] FIG. 6 is a cross-section taken on the line VI-VI in FIG.
5,
[0020] FIG. 7 shows yet another electrophoretic display device
according to the invention, while
[0021] FIG. 8 shows how different grey values (intermediate optical
states) have been realized in the display device of FIG. 8, and
[0022] FIG. 9 shows a variant of FIG. 7.
[0023] The Figures are diagrammatic and not drawn to scale;
corresponding parts are generally denoted by the same reference
numerals.
[0024] FIG. 1 shows an electric equivalent of a part of a display
device 1 to which the invention is applicable. It comprises a
matrix of pixels 10 at the area of crossings of row or selection
electrodes 7 and column or data electrodes 6. The row electrodes 1
to m are consecutively selected by means of a row driver 4, while
the column electrodes 1 to n are provided with data via a data
register 5. To this end, incoming data 2 are first processed, if
necessary, in a processor 10. Mutual synchronization between the
row driver 4 and the data register 5 takes place via drive lines
8.
[0025] Drive signals from the row driver 4 and the data register 5
select a pixel 10 (referred to as passive drive). In known devices,
a column electrode 6 acquires such a voltage with respect to a row
electrode 7 that the pixel assumes one of two extreme states at the
area of the crossing (for example, black or colored, dependent on
the colors of the liquid and the electrophoretic particles).
[0026] If desired, drive signals from the row driver 4 may select
the picture electrodes via thin-film transistors (TFTs) 9 whose
gate electrodes are electrically connected to the row electrodes 7
and whose source electrodes 21 are electrically connected to the
column electrodes 6 (referred to as active drive). The signal at
the column electrode 6 is transferred via the TFT to a picture
electrode, coupled to the drain electrode, of a pixel 10. The other
picture electrodes of the pixel 10 are connected to, for example,
ground, for example, by means of one (or more) common counter
electrode(s). In the example of FIG. 1, such a TFT 9 is shown
diagrammatically for only one pixel 10.
[0027] In a display device according to the invention, each pixel
is provided with a further electrode and drive means for supplying
the further electrode with electric voltages. This is shown in FIG.
2, in which a cross-section of such a pixel provided with a third
electrode 6' is shown. The drive means comprise, for example, the
data register 5 (and possibly a part of the driver), and extra
column electrodes 6' (and an extra TFT in the case of active
drive).
[0028] A pixel 10 (FIG. 2) comprises a first substrate 11, for
example, of glass or a synthetic material, provided with a
switching electrode 7, and a second, transparent substrate 12
provided with a switching electrode 6. The pixel is filled with an
electrophoretic medium, for example, a white suspension 13
containing, in this example, positively charged, black particles
14. The pixel is further provided with a third electrode 6' (and,
if necessary, as described above, with drive means not shown in
FIG. 2) so as to realize intermediate optical states via electric
voltages across the third electrode. In this respect, it should be
noted that the third electrode 6' also influences the switching
behavior between the two extreme states. As will be further
explained hereinafter, the voltage across this electrode also
influences the two extreme states.
[0029] For example, in FIG. 2A, the switching electrode 7 is
connected to ground, while both electrodes 6, 6' are connected to a
voltage +V. The black particles 14 (positively charged in this
example) move towards the electrode at the lowest potential, in
this case the electrode 7. Viewed from the viewing direction 15,
the pixel now has the color of the liquid 13 (which is white in
this case). In FIG. 2B, the switching electrode 7 is connected to
ground, while both electrodes 6, 6' are connected to a voltage -V.
The positively charged, black particles 14 move towards the lowest
potential, in this case towards the potential plane defined by the
electrodes 6, 6', parallel to and just alongside the substrate 12.
Viewed from the viewing direction 15, the pixel now has the color
of the black particles 14.
[0030] Also in FIG. 2C, the switching electrode 7 is connected to
ground. The electrode 6 is again connected to a voltage -V.
However, similarly as electrode 7, the third electrode 6' is now
connected to ground. The positively charged, black particles 14
move towards the lowest potential, in this case an area around
electrodes 6. This is even more strongly the case when the third
electrode 6' is connected to a voltage +V, as is shown in FIG. 2D.
Viewed from the viewing direction 15, the pixel now has only partly
the color of the black particles 14 and partly the color of the
white liquid. A grey hue is thereby obtained (dark grey in the case
of FIG. 2C and light grey in the case of FIG. 2D). This will be
explained with reference to FIG. 3 in which the potential lines are
shown for six possible combinations of voltages across the
electrodes 6, 6', 7, and arrows 16 diagrammatically show the
direction of the electric forces exerted on the particles 14.
[0031] Since the particles may not remain positioned on the
substrate, for example, due to movement in the liquid, it may be
advantageous to provide it with a sticking layer.
[0032] Another possibility of limiting the movement of the liquid
is the use of microcapsules as described in "Micro-encapsulated
Electrophoretic Materials for Electronic Paper Displays", 20.sup.th
IDRC conference, pp. 84-87 (2000). The electrophoretic medium, a
liquid 13 containing positively charged particles 14, is now
present in microcapsules 17 in a transparent substrate 18 (see FIG.
4).
[0033] In FIG. 4A, the switching electrode 7 is again connected to
ground (0 V), while the electrodes 6, 6' are again connected to a
voltage +V. The positively charged, black particles 14 move towards
the electrode at the lowest potential, in this case the electrode
7, i.e. towards the lowest part of the microcapsule 17. Viewed from
the viewing direction 15, the pixel again has the color of the
liquid 13. In FIG. 4B, the switching electrode 7 is connected to
ground, while both electrodes 6, 6' are connected to a voltage -V.
Viewed from the viewing direction 15, the pixel now has the color
of the black particles 14.
[0034] Also in FIG. 4C, the switching electrode 7 is connected to
ground. The electrode 6 is again connected to a voltage -V.
However, similarly as electrode 7, the third electrode 6' is now
connected to ground. The positively charged, black particles 14
move towards the lowest potential, in this case towards electrode 6
and are eventually largely present in the upper part of the
microcapsule 17. Viewed from the viewing direction 15, the pixel
now has a dark grey color. When the third electrode 6' is connected
to a voltage +V, as is shown in FIG. 4D, the particles 14 are
eventually present along an edge of the microcapsule 17. The pixel
now has a light grey color.
[0035] In the display device of FIG. 5, the switching electrodes 6,
7 and the third electrode 6' are present on the same substrate 11,
while the third electrode is separated from the switching
electrodes by means of a layer 18 of a dielectric material. In this
example, the switching electrodes 6, 7 are comb-shaped and
interdigital, and parts of the third electrode 6' are situated
between the teeth of the two switching electrodes. The comb shape
is not strictly necessary. A lateral field between the two
switching electrodes 6, 7 is also satisfactory. The cross-section
shown in FIG. 6 may then depict both a complete pixel and a part of
a pixel of the display device of FIG. 5. Illustrated in the same
way as in FIGS. 2, 3, various electric field configurations can be
introduced again, with the attendant different grey values. To
prevent mixing of particles 14 belonging to two pixels, walls or
barriers 19 may be provided. When a plurality of colors is used, it
is often desirable to provide these walls or barriers (either or
not across the full height of the pixel).
[0036] The electrophoretic medium may also be present in a
prismatic structure as described in "New Reflective Display Based
on Total Internal Reflection in Prismatic Microstructures", Proc.
20.sup.th IDRC conference, pp. 311-314 (2000). This is shown in
FIGS. 7, 8. The known device comprises a prismatic structure of (in
this example) a repetitive structure of hollow (for example, glass)
triangles comprising a liquid 13 which contains positively charged
particles. Dependent on the voltages across the electrodes 6, 7,
the positively charged particles are present on the (bottom)
electrode 7 of metal or on the ITO (top) electrode 6. In the
first-mentioned case, an incident beam undergoes total reflection
on the glass-liquid interface and is reflected (arrow a). In the
second case, an incident beam is absorbed on the glass-liquid
interface (arrow b).
[0037] By introducing a third electrode 6' again, various electric
field configurations with the attendant different grey values may
be introduced again, similarly as in the examples of FIGS. 2 and 4.
When light-absorbing, positively charged particles 14 are used in a
liquid 13, the configurations 8A, 8B, 8C and 8D correspond to the
colors white, black, dark grey and light grey. By introducing a
further electrode 6", with the electrodes 6', 6" being supplied
with different voltages, intermediate grey values can be realized
(see FIG. 8E).
[0038] The invention is of course not limited to the examples
described above. For example, four possible grey hues are obtained
in the examples described above. It will be evident that a
plurality of grey hues can be displayed at varying voltages and
that a full grey scale can be realized in an analog manner. Several
variations are also possible for the shape of the prismatic
structure of FIG. 8, such as roof-shaped, spherical or cylindrical
structures, as is shown diagrammatically in a cross-section, by way
of example, in FIG. 9. Alternatively, the liquid 13 may be filled
with a black ink containing white TiO.sub.2 particles 14. To
prevent crosstalk between pixels, the pixels should be electrically
shielded from each other as much as possible. This may be realized
by providing walls having a very high dielectric constant, or by
conducting walls. The conducting walls may be connected to
electrode 7 (ground).
[0039] A combination of one or more of said possibilities is
alternatively applicable in practice.
[0040] The protective scope of the invention is not limited to the
embodiments described.
[0041] The invention resides in each and every novel characteristic
feature and each and every combination of characteristic features.
Reference numerals in the claims do not limit their protective
scope. Use of the verb "to comprise" and its conjugations does not
exclude the presence of elements other than those stated in the
claims. Use of the article "a" or "an" preceding an element does
not exclude the presence of a plurality of such elements.
* * * * *